Abstract
There are three optical in-water components that, besides water itself, govern the under-water light field: phytoplankton, total suspended matter (TSM) and colored dissolved organic matter (CDOM). In essence, it is the spectral absorption and scattering properties of each optical component that govern the underwater light field, and also the color of the sea that we can perceive, and that can also be measured remotely from space. The Baltic Sea is optically dominated by CDOM, apart from cyanobacteria blooms that often cover most of the Baltic proper during summer. Remote sensing images of TSM reveal large-and mesoscale features and currents, especially in the Southern Baltic, which are influenced both by atmospheric Rossby waves and the Coriolis force. In coastal waters, the optical properties are strongly influenced by inorganic suspended matter, which may originate from coastal erosion and from run-off from land, streams and rivers. In this paper, we evaluate the distribution of TSM across the Baltic Sea using remote sensing data and statistically compare the TSM loads in the different Helsinki Commission (HELCOM)-defined basins. The TSM loads during summer vary substantially in the different basins, with the south-eastern Baltic overall being most influenced by cyanobacteria blooms. The Gdansk basin and the Gulf of Riga were distinguished both by relatively high TSM loads with high standard deviations, indicating strong fluvial input and/or resuspension of sediments. We also evaluate a coastal TSM transect in Himmerfjarden bay, which is located at the Swedish East coast in the Western Gotland Basin. The effect of wind-wave stirring on the distribution of TSM from source (shore) to sink (open sea) can be assessed using satellite data from European Space Agency’s (ESA) MEdium Resolution Imaging Spectrometer (MERIS) mission (2002-2012) with 300 m resolution. The TSM transect data from areas with low wind exposure and a stable thermocline showed a gradient distribution perpendicular to the coast for summer seasons 2009, 2010, 2011 and a 3-year summer composite, confirming a previous bio-optical study from the Western Gotland basin.
Highlights
Marine waters are usually divided into optical case 1 and optical case 2 waters (Morel and Prieur, 1977)
The 3-year composite averaging all viable summer data (2009–2011) shows the distribution of total suspended matter (TSM) corresponding to the previous Helsinki Commission (HELCOM) assessment period which spanned over a five-year period (2007–2011)
The problems related to frequent cloud cover in the Baltic Sea, leading to limited coverage of individual MEdium Resolution Imaging Spectrometer (MERIS) scenes, could be overcome by producing averaged summer composite using the L3 binning approach
Summary
Marine waters are usually divided into optical case 1 and optical case 2 waters (Morel and Prieur, 1977). Optical case 2 waters are coastal waters that are influenced by terrestrial run-off, and are optically influenced by total suspended matter (TSM, termed suspended particulate matter, SPM) and by CDOM. There is a strong influence of TSM due to both high river run-off and tidal influence Examples for this are the Thames Estuary where, according to Devlin et al (2008) estuarine waters typically have much higher concentrations of TSM (8.2–73.8 gm−3) compared to coastal waters (3.0–24.1 gm−3). Off-shore waters showed concentrations of up to 9.3 gm−3 which is much higher than found in the Baltic Sea. Kratzer and Tett (2009), for example found that in summer (Western Gotland Basin) the measured TSM ranged from only 0.48–1.34 gm−3 in the open sea (n = 18) and 0.48–2.77 gm−3 in coastal areas (n = 22). The Rio de Plata at the border between Argentina and Uruguay has shown to have very high turbidity ranging from 62–185 NTU, and the Gironde Estuary in southwest France showed ranges of 41–988 FNU (Dogliotti et al, 2015)
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